1. Field of the Invention
The present invention relates to a type of electrophotographic photosensitive member wherein its functions are allotted to a charge generation layer and a charge transport layer. More particularly, it relates to an electrophotographic photosensitive member containing a specific disazo pigment in its charge generation layer as well as a specific hydrazone compound in its charge transport layer.
2. Description of the Prior Art
There have so far been known selenium, cadminum sulfide, zinc oxide, etc. as photoconductive materials for use in electrophotographic photosensitive members. In contrast to the many advantages thereof, such as, for instance, chargeability in the dark to a suitable potential, small dissipation of charge in the dark, and fast dissipation ability by light irradiation, these photoconductive materials have the disadvantages of lacking film forming property per se with very few exceptions such as amorphous selenium and of poor ability to retain a charge applied to their surface.
On the other hand, a variety of organic photoconductive materials are known, including photoconductive polymers such as polyvinylcarbazole and polyvinylanthracene.
There are also known low-molecular organic photoconductors such as pyrazoline compounds disclosed in British Pat. No. 1030024 and U.S. Pat. No. 3,180,729 and styryl compounds disclosed in British Pat. Nos. 1465141 and 1465142.
However, these organic photoconductive materials cannot be said to be useful in practice since they generally have neither enough sensitivity nor sufficient film forming property in actual use.
Also, although these low-molecular organic photoconductors have solved the problem of film forming property, which has been an objection to the application of the above organic photoconductive polymers, by their selection of a suitable binder, there are still many difficulties in their practical applications because of their low sensitivity.
In view of the above, a photosensitive member of a laminated structure has been recently proposed which comprises two photosensitive layers, a charge generation layer and a charge transport layer, having allotted functions. The electrophotographic photosensitive member having such photosensitive layers in a laminate structure has been improved in sensitivity to visible light, in charge retentivity, and in surface strength.
Such photosensitive members are disclosed in U.S. Pat. Nos. 3484237, 3837851, and 3871882 and British Pat. No. 1453024.
It has been known that the following substances can be incorporated into the charge generation layer of the above-mentioned electrophotographic photosensitive member having a laminate structure comprising photosensitive layers for charge generation and charge transport which have those respective allotted functions: inorganic substances such as selenium, selenium-tellurium, selenium-arsenic, cadmium sulfide, and amorphous silicon and organic substances such as pyrylium dyes, thiopyrylium dyes, triarylmethane dyes, thiazine dyes, cyanine dyes, phthalocyanine pigments, perylene pigments, indigo pigments, thioindigo pigments, quinacridone pigments, squaric acid pigments, azo pigments, polycylic quinone pigments, etc.
For example, disazo pigments having a skeleton of 2-phenyl-benzoxazole have been proposed as a charge-generating material for use in a charge generation layer (Ishikawa et. al., U.S. Pat. Ser. No. 235,793 filed Feb. 19, 1981; Takasu et al., Japanese Patent Application No. 178532/1980).
For the charge transport layer, it has been disclosed to incorporate thereinto pyrene, N-ethylcarbazole, N-isopropylcarbazole, 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, pyrazoline compounds such as 1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline, hydrazone compounds such as p-diethylaminobenzaldehyde-N,N-diphenylhydrazone, oxazole compounds such as 2-(p-diethylaminostyryl)-6-diethylaminobenzoxazole, triphenylamine, poly(N-vinylcarbazole), halogenated poly(N-vinylcarbazole), polyvinylpyrene, polyvinylanthracene, polyvinylacridine, poly(.alpha.-vinylphexylanthracene), pyrene-formaldehyde resin, ethylcarbazole-aldehyde resin, etc.
However, satisfactory sensitivity or durability has not yet been obtained with electrophotographic photosensitive members having any of the above-noted compounds in each photosensitive layer. In photosensitive members of the present type, their sensitivity and endurance characteristics vary depending upon combinations of charge-generating and charge-transporting materials used in the charge generation and charge transport layers, respectively. Although a general method is known to select a charge-transporting material taking its ionization potential into consideration as a measure for evaluating it, in practice, combinations of charge-generating and charge-transporting materials are now selected only after repetition of a great number of experiments. Additionally, the photosensitive members of the laminate type have a relatively large photomemory effect as compared with photosensitive members having inorganic photoconductors. This is also a problem which seeks a solution.